1. Field of the Invention
The present invention relates to a mass spectrometer using plasma as anion source, and more particularly, to amass spectrometer having an ion deflector lens.
2. Description of the Related Art
In a mass spectrometer using plasma as an ion source, such as an inductively coupled plasma mass spectrometer (ICP-MS) or a microwave plasma mass spectrometer (MIP-MS), it has been required that photons and neutral particles with high energy, which cause background noises, be separated from ion beams (for example, refer to JP 10-97838 A (FIG. 10), JP 2004-71470 A (FIG. 1), JP 8-7829 A (Paragraph [0009]), JP 2002-525821 A (FIG. 1), JP 61-107650 A (FIGS. 1 and 2), JP 2000-67805 A (FIG. 1), and JP 2000-100375 A (FIG. 2)).
Conventionally, photons and neutral particles which cause the background noises are separated from ion beams by an aid of a tilted or bent ion guide (for example, refer to JP 10-97838 A). Hereinafter, the tilted or bent ion guide is generally called “nonlinear ion guide.” Photons and neutral particles are also separated from ion beams by an aid of a cylindrical ion deflector lens. The cylindrical ion deflector lenses are of a configuration symmetrical with respect to a point (for example, refer to JP 2004-71470 A) or of a configuration asymmetrical with respect to a point so as to provide an angle between before and after the ion deflector lens in an ion flying direction (for example, refer to JP 8-7829 A). Herein, the point asymmetrical means asymmetrical with respect to a point. Photons and neutral particles are also separated from ion beams by means of an ion mirror (for example, refer to JP 2002-525821 A and JP 2000-67805 A).
When the nonlinear ion guide is used for separation of photons and neutral particles, ion beams cannot be precipitously deflected by the nonlinear ion guide. Therefore, when an interval between an aperture arranged in front of the nonlinear ion guide and an aperture arranged at the other end of the ion guide is narrow, because the deviation quantity of ion beam becomes small, blocking of photons and neutral particles in the nonlinear ion guide is not sufficient. In addition, an input part and an output part of the tilted ion guide as well as a bent portion of the bent ion guide are low in the transmittance of ion.
Further, in the case of using an ion deflector lens for separation of photons and neutral particles, when the deviation quantity of ion beam is reduced, it is necessary to also reduce the diameter of the ion deflector lens. As a result, an aberration of the deflected ion beam is increased, and the transmittance of ions is reduced. Besides, the ion deflector lens that provides an angle between upstream and downstream of an ion flying direction is a factor for increasing the difficulty in the manufacture of the front and rear ion optical systems, and also a factor for increasing the size of a mass spectrometer.
Further, in the case of using an ion mirror for separation of photons and neutral particles, a deflector mechanism for the ion beam is complicated and large in size. Further, in order to set the transmittance of ion beam to a practical value, a plurality of electrode voltages are required to be adjusted.